Comparison of carbon storage under aerobic and anoxic conditions

In various activated sludge systems, the biomass grows under transient (unb alanced) conditions and the storage response (formation of internal polymer s as the fastest adaptation to the changing environment) becomes important. Till now the role of storage on population dynamics has been deeply invest igated under anaerobic (EBPR processes) or aerobic (bulking control) condit ions. Little attention has been given to processes including anoxic conditi ons even though in many of them storage phenomena are likely to occur (anox ic selectors, nitrogen removal processes with addition of an external sourc e of readily biodegradable COD or with aerobic contact/anoxic stabilization ). For these reasons, the aim of the present work was to investigate storag e and succeeding use of stored products under anoxic and mixed (anoxic/aero bic) conditions. Batch experiments have shown that a mixed culture selected under aerobic conditions and intermittent feed (acetate-limited medium), w as also able to take up acetate (90-100 mgCOD/gCOD h) and store it as PHB ( 35-40 mgCOD/gCOD h) under anoxic conditions. After acetate depletion, the s tored PHB was used for growth and maintenance. The NUR on acetate in the pr esence of storage was 20 mgN/gVSS h (which corresponded to a COD removal of 6.9 mgCOD/mgN) while it dropped to 10-3 mgN/gVSS h in the "endogenous phas e" when denitrification was on the stored PHB. The presence of aerobic cond itions instead of anoxic ones had a major positive effect on the rate and y ield of PHB storage while it had only a minor effect on the rate of PHB con sumption. The latter observation can be explained by assuming that the hydr olysis of the stored product is the rate limiting step of the "endogenous" metabolism and that the hydrolysis rate is not highly dependent on aerobic- anoxic conditions. Cross-comparison of PHB storage and consumption under ae robic/anoxic conditions made it possible to determine that, in the particul ar mixed culture under investigation, all aerobic heterotrophs able to stor e were also able to denitrify. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.